Sensors are often used to measure properties of printer fluid within a printer fluid tank or other receptacle. Such sensors can, for example, include temperature sensors to measure a temperature of the printer fluid, conductivity sensors to measure conductivity of the printer fluid, volume sensors to measure a volume of the printer fluid within a receptacle, and light sensors to measure optical properties of the printer fluid. Readings from such sensors can be used to determine whether the printer fluid is appropriate for use in a printer or other appliance. For example, printer fluid that is stored at too high of a temperature may result in undesirable printing performance. Readings from a light sensor can, for example, be used to determine a density of the printer medium based on solid contents within a carrier fluid of the printer medium. Based on readings from one or more sensors, an appliance can alert an operator of undesirable conditions, and in some appliances can automatically take corrective action to adjust properties of the printer medium.